Disc brakes have come to predominate in automobile braking systems and are employed in far greater numbers than drum brakes. However, the disc brake rotors require accurate facing and finishing in order to avoid poor or unsatisfactory braking performance. The advent of disc brakes accordingly requires brake lathes which more accurately control positioning of cutting and finishing tools than were required when boring and finishing brake drums.
One form of brake lathe which has proved quite satisfactory is disclosed in U.S. Pat. No. 4,506,570 (which is incorporated herein by this reference). That lathe positions a disc brake rotor on an arbor held in place by a bell clamp and a number of spacers as desired. A saddle, upon which tool posts are pivotally mounted for receiving cutting and finishing tools, is positioned using a cross slide mechanism. A first, longitudinal dovetail way oriented parallel to the axis of the arbor, carries a second, transverse dovetail way on which the saddle is mounted. Feedscrews connected to the ways in rotating and threaded fashion are employed to control longitudinal and transverse positioning of the saddle, and therefore, the cutting and finishing tools, with great accuracy. Power sources may be connected to each feedscrew via clutches and gearboxes for automatic positioning, cutting and finishing.
The longitudinal and transverse ways of these conventional brake lathes are relatively massive and require great care, effort and thus expense to manufacture and machine properly in order to obtain the close tolerances required to face brake rotors properly, and to be sufficiently durable. Additionally, over the life span of the lathe, the ways slide relative to each other only a fractional portion of their length, so that they wear unevenly. Moreover, the length of the dovetail ways permits filings, shavings and other debris from the cutting and finishing process to become interposed in the ways and create additional wear and inaccuracy in positioning of the tools.
It is also the case that on conventional brake lathes which feature the cross slide mechanism for positioning the cutting or finishing tool, considerable time and effort are required for changeover from drum boring to rotor facing configuration, and vice versa. Thus, for example, changeover from rotor facing to drum boring configuration requires removal of the saddle, tool bars and tools that are employed for facing rotors, bolting on or otherwise attaching on the cross slide mechanism the saddle that is employed for boring drums, and then making all necessary adjustments to ensure that the tool is properly placed and all fasteners have been properly tightened into place in order to avoid inadvertent movement of the drum cutting tool. Equipping a lathe with two cross slide mechanisms to eliminate this changeover problem would be inordinantly expensive in the price-competitive brake lathe market.
Another concern raised by drum boring operations on conventional brake lathes is that the operator finds it difficult to see the tool as it works the inner cylindrical surface of the drum. The cross-slide mechanism is located on the side of the lathe where the hand wheels and electrical controls are found. Therefore, where the operator is stationed, the boring bar that extends from the drum boring saddle extends into the drum and works toward the operator against the interior drum surface. This geometry makes it more difficult to set the tool up initially for finishing the drum, and even more difficult to monitor the finishing operation.